The issue is with the rear brake linkage, which makes it so that instead of a 150mm hub, the gemini DH uses a 135mm hub. All you need to do is remeasure your hub dimensions taking this into account and you're set.

There's a lot of things that go into getting the spokes right on a wheelset. If you still have the old wheelset take it down to your local LBS and they can measure the spokes you've got. Otherwise you need to use a spoke calculator (like the one on DT Swiss website) to figure it out. Check out the wheels and tires forum you will find a lot more useful information in there on the subject.